This invention relates to integrated circuits, and specifically to a decoder circuit which responds to a multistate control signal received through a single input terminal, and to a clock signal, by producing a select signal at an output terminal which corresponds to the state of the control signal.
Very large scale integrated (VLSI) circuits include many circuit functions in one integrated circuit (IC) chip. Chips are encased in packages with terminals or "pins" along the sides. Signals are input and output for these circuit functions through the pins. As chip area increases at a given circuit (function) density, or as the density increases, the ratio of side length/functions decreases. Difficulties in accommodating the number of input-output (I/O) pins required for exchanging signals with such a VLSI circuit are therefore common. Using larger chips with more electrical contacts to external pins, and packages with more pins, in effect decreases the circuit density, and is unsatisfactory.
In one prior art approach to the pin scarcity problem, three or more discreet signal levels are used on an I/O pin. Detecting the third, intermediate level signals requires more complex and precise circuitry than detecting binary extremes. In another approach, signals are time multiplexed using serial data words and a clock to increment a counter which, for each bit of data word, sequentially selects an output line or mode of operation. The output lines are selectable sequentially in rotation, not randomly. These techniques are relatively complex and involve substantial system overhead.
A third prior art technique, disclosed for example in U.S. Pat. No. 4,465,944 to Shin, uses three-state signals. Three signals, only one of which will be "on" at a time, are combined into a three-state control signal having a value of either high voltage, low voltage, or "open" (high impedance). In the first two cases, the control signal is used by the input decoder circuit to override high and low reference signals, to constant high or low states. In the third case, the control signal is essentially disconnected and does not affect the reference signals. Still, there is a need for a more convenient way to communicate more than three exclusively selectable signals through a single pin.